1. Field of the Invention
Example embodiments of the present invention are directed generally to a phase locked loop and method thereof, and more particularly to a phase locked loop configured to selectively operate in one of a plurality of frequency zones and method thereof.
2. Description of Related Art
A conventional phase locked loop (PLL) may be a frequency feedback circuit generating a signal having an arbitrary frequency in response to a frequency of a signal input from an external source. The PLL may synchronize phases of reference and oscillator signals by detecting a phase difference between the reference and oscillator signals and adjusting the frequency of the oscillator signal to a given frequency using an up/down signal based on the detected phase difference. PLLs may be deployed within frequency synthesis circuits and clock recovery circuits used in data processing circuits.
FIG. 1 is a block diagram of a conventional PLL. As illustrated in FIG. 1, the PLL may include a divider 1, a phase frequency detector (PFD) 2, a charge pump 3, a loop filter 4, a voltage-controlled oscillator 5 and a divider 6.
Referring to FIG. 1, the divider 1 may divide the frequency of a reference signal Fref and may output the frequency-divided signal to the phase frequency detector 2. The phase frequency detector 2 may receive the frequency-divided signal from the divider 1 and an oscillator signal fed back from the voltage-controlled oscillator 5 as inputs, may compare the phases and frequencies of the two signals, and may output a signal based on the comparison result to the charge pump 3.
Referring to FIG. 1, the charge pump 3 may provide current corresponding to the comparison result output from the phase frequency detector 2 to the loop filter 4. The loop filter 4 may filter an output signal of the charge pump 3 and may provide a voltage signal Vlf to the voltage-controlled oscillator 5.
Referring to FIG. 1, the voltage-controlled oscillator 5 may generate an oscillator signal Fvco having a given frequency corresponding to the voltage signal Vlf output from the loop filter 4 and may provide the generated oscillator signal Fvco to an external circuit (not shown). The divider 6 may divide the oscillator signal Fvco output from the voltage-controlled oscillator 5 and may output the divided signal to another input terminal of the phase frequency detector 2 as a feedback signal.
Referring to FIG. 1, the voltage-controlled oscillator 5 may select one of the plurality of a frequency zones in which to operate as an operating frequency zone in response to a given control signal. In addition, the operating frequency zone may adjusted in response to a change of the control signal.
Referring to FIG. 1, the PLL may further include a frequency detector 7, a look-up table 8 and a digital-analog converter (DAC) 9. The frequency detector 7 may detect the frequency of the oscillator signal Fvco and may output a signal based on the result of the detection to the look-up table 8. The lookup table 8 may output, to the digital-analog converter 9, a given N-bit zone control signal N-BIT DATA in response to the signal output from the frequency detector 7 to the digital-analog converter 9. The digital-analog converter 9 may convert the received N-bit zone control signal (N-BIT DATA) into an analog signal and may output the analog signal to the voltage-controlled oscillator 5. The voltage-controlled oscillator 5 may select the operating frequency zone corresponding to the analog signal output from the digital-analog converter 9. Accordingly, the voltage-controlled oscillator 5 may generate an oscillator signal Fvco having a frequency based on the voltage signal Vlf in response to a characteristic graph of the selected operating frequency zone. A resistor R and a capacitor C illustrated in FIG. 1 may constitute a lower-pass filter, which may enable a slower shift from one operating frequency zone to another in response to control signal change.
FIG. 2 is a graph illustrating voltage-frequency characteristics based on an operation of the conventional PLL of FIG. 1. In the graph in FIG. 2, the horizontal axis may denote a voltage signal Vlf input to the voltage-controlled oscillator 5, and the vertical axis may denote the frequency of the oscillator signal Fvco of the voltage-controlled oscillator 5. In particular, FIG. 2 illustrates the plurality of operating frequency zones within which the voltage-controlled oscillator 5 may operate. For example, the plurality of frequency zones may include sixty four frequency zones Z1 to Z64 and the N-bit zone control signal (N-BIT DATA) may include 6 bits to distinguish between the sixty four frequency zones.
Referring to FIGS. 1 and 2, the frequency of the oscillator signal Fvco of the voltage-controlled oscillator 5 may be increased as the level of the voltage signal Vlf input to the voltage-controlled oscillator 5 increases. The N-bit zone control signal (N-BIT DATA) corresponding to a frequency measured using the look-up table 8 may be output to the digital-analog converter 9 if the frequency of the oscillator signal Fvco is greater than or equal to a given threshold value or, alternatively, less than or the given threshold value. The voltage-controlled oscillator 5 may select a different frequency zone if the N-bit zone control signal (N-BIT DATA) is changed, and may thereby generate the oscillator signal Fvco at a frequency corresponding to the voltage signal Vlf (e.g., as shown in the graph of FIG. 2).
However, in the conventional PLL of FIG. 1, a range associated with a given frequency step or zone of the frequency zones may change due to variations in process, voltage, temperature, etc. Accordingly, the frequency zones may not remain constant during operation. Accordingly, the shape of the frequency graph of the oscillator signal Fvco corresponding to the level of the voltage signal Vlf may change.
In order to reduce frequency zone fluctuation, a calibration process may be used. In the calibration process, each of the plurality of frequency zones may be measured at an initial PLL calibration, and the measured frequency zones may be stored to calibrate the PLL. However, the time required to calibrate the PLL may increase and an accuracy of the calibration decreases (e.g., during an operation of the PLL). Further, it may be difficult to re-calibrate the PLL during an operation of the PLL. Also, if a gain Kvco of the voltage-controlled oscillator becomes non-linear due to a change in the shape of the frequency graph, it may be more difficult to detect the frequency zones, and it may thereby become more difficult to synchronize the phase of an output signal to the reference signal.